1. Field of the Invention
This invention relates to a radiation image formation system and, more particularly, to a radiation image formation system for reducing a magnified radiation image radiographed into life size and displaying the reduced radiation image.
2. Description of the Related Art
In recent years, a radiation image radiographing apparatus for radiographing a phase contrast image has been proposed. A phase contrast image is also called a refraction contrast image and used to be said that it could be obtained by radiographing with monochromatic parallel X-rays emitted from a synchrotron radiation X-ray source, such as Spring-8, or with a micro focus X-ray source having a focus size of about 10 μm. However, it has turned out that a phase contrast image can be obtained with a medical X-ray source (see Japanese Patent Application Publication (Laid-Open) No. 2001-91479). A phase contrast image is sharper and more high-resoluted than an ordinary absorption contrast image.
Such a phase contrast image is obtained in the following way. A subject is located apart from a radiation image detector for detecting X-rays which transmit through the subject. When the subject is radiographed, X-rays which transmit through the subject are refracted and the density of X-rays inside of the border of the subject becomes low. On the other hand, the density of X-rays outside of the border of the subject becomes high because X-rays which do not transmit through the subject and X-rays which transmit through the subject overlap. As a result, an edge, which is the border of the subject, is enhanced.
By the way, when a subject located apart from a radiation image detector is radiographed, the size of a radiation image obtained is larger than that of the subject. However, it is convenient, for example, for a doctor who makes a medical diagnosis to use a radiation image in life size. Therefore, preferably, a subject is magnified and is radiographed to obtain a phase contrast image. After that, the image reduced at the reducing rate 1/M, which corresponds to the magnifying rate M, is output. By doing so, the image in life size is recorded on a recording medium with a recording device.
Conventionally, a radiation image formation system in which radiography is performed by the use of a radiation image radiographing apparatus where a distance between a subject platform and a radiation detector can be adjusted freely, a magnifying rate is calculated from the distance between the subject platform and the radiation detector and then an image is reduced on the basis of this magnifying rate and output to an image output apparatus, is disclosed as a radiation image formation system for forming a phase contrast image as an image in life size (see Japanese Patent Application Publication (Laid-Open) No. 2001-238871, Paragraph Nos. [0061],[0064], and [0065], FIG. 7, etc.).
With such a conventional radiation image formation system (see Japanese Patent Application Publication (Laid-Open) No. 2001-91479), however, relations among an input pixel size by which a radiation image is converted into digital image data, a minimum output pixel size by which an image output apparatus outputs an image, and a magnifying rate at which a radiation image is radiographed are not taken into consideration. Accordingly, radiographing may be performed at a magnifying rate which satisfiesmagnifying rate>input pixel size/minimum output pixel size
In this case, since a reducing rate is limited by the minimum output pixel size, an image to be output cannot be reduced in life size.
For example, if the input pixel size is 200 μm and the minimum output pixel size is 100 μm, an image to be output can be reduced at a reducing rate up to 1/2. When a radiation image is radiographed at the magnifying rate of, for example, 2, if the radiation image to be output is reduced at the reducing rate of 1/2, an image in life size can be obtained. Therefore, if the input pixel size is 200 μm and the minimum output pixel size is 100 μm, an image in life size can be output by performing radiographing at the magnifying rate of 2 and by reducing a radiation image obtained at the reducing rate of 1/2. However, an image in life size cannot be obtained when radiography is performed at the magnifying rate of 4. This is because a radiation image radiographed cannot be reduced at a reducing rate of 1/4.